1. Field of the Invention
The present invention relates to circuits, apparatuses, and systems, which drive capacitive loads such as charge coupled devices (CCDs). More particularly, the present invention relates to a drive circuit and a driving method for a charge transfer device and a charge transfer system, which are capable of reducing interference between two or more drive voltages that are applied.
2. Description of the Related Art
In recent years, there is a growing request that images be picked up at higher speed, regardless of television (TV) systems, to slow down the playback speed in video cameras having CCDs mounted therein. In addition, the digital cameras having CCDs mounted therein has a problem in that the transfer speeds are reduced with the increasing number of pixels. As a result, it is desired to provide high-speed imagers.
FIG. 12A shows an example of the structure of a CCD imaging apparatus in related art and FIG. 12B illustrates how to drive the CCD imaging apparatus. FIG. 12A shows an example of the structure of the main part of an imaging apparatus 30 using an interline transfer (IT) CCD. The imaging apparatus 30 in FIG. 12A includes multiple photosensors 31, which are pixels, arranged in an array form. The imaging apparatus 30 also includes multiple CCD vertical transfer registers 33 for every column of the photosensors 31 and a CCD horizontal transfer register 34 that is connected to the bottoms of the vertical transfer registers 33. An output unit 36 is connected downstream of the horizontal transfer register 34.
Two vertical transfer electrodes are provided for every photosensor 31 in the vertical transfer registers 33. Signal charge is vertically transferred and driven by using four-phase vertical drive pulses φV1, φV2, φV3, and φV4. Specifically, the vertical drive pulses φV1, φV2, φV3, and φV4 are applied to four vertical transfer electrodes 32 (321, 322, 323, and 324), respectively. The vertical transfer electrodes 321 and 323 are arranged at positions corresponding to the photosensors 31. The bottom photosensors 31 correspond to the vertical transfer electrode 321 to which the vertical drive pulse φV1 is applied.
The vertical transfer registers 33 are connected to the horizontal transfer register 34 via the vertical transfer electrodes 321 to 324 corresponding to one bit in the bottom, to which the vertical drive pulses φV1 to φV4 are applied. The vertical transfer electrodes 321 to 324 horizontally extend so as to be shared between the vertical transfer registers 33.
Two horizontal transfer electrodes 35 (351 and 352) are provided for every vertical transfer register 33 in the horizontal transfer register 34. Signal charge is horizontally transferred and driven by using two-phase horizontal transfer pulses φH1 and φH2.
In the imaging apparatus 30, the photosensor 31 detects light, performs photoelectric conversion to the detected light, and accumulates the signal charge corresponding to the amount of the detected light. The signal charge in the photosensor 31 is read out from the photosensor 31 to the vertical transfer register 33 during a vertical blanking interval and, then, the signal charge for every horizontal line is vertically transferred during a horizontal blanking interval (hereinafter referred to as “vertical line shift”) to transfer the signal charge to the horizontal transfer register 34. The signal charge transferred to the horizontal transfer register 34 is horizontally transferred during a horizontal scanning period and is output through the output unit 36.
In the vertical line shift in the CCD in the related art, the signal charge is transferred and driven by using the vertical drive pulses φV1 to φV4 during a horizontal blanking interval Hb in the TV system. FIG. 12B shows driving timings in the vertical line shift. As shown in FIG. 12B, in the vertical line shift, the signal charge held in, for example, the vertical transfer electrodes 322 and 323 corresponding to the vertical drive pulses φV2 and φV3 is vertically shifted to the horizontal transfer register 34 by using the four-phase vertical drive pulses φV1 to φV4 during the horizontal blanking interval Hb.
Specifically, the signal charge is transferred to the horizontal transfer electrode 351, to which the horizontal drive pulse φH1 is applied, in the horizontal transfer register 34 at a falling edge of the vertical drive pulse φV4 to be applied to the vertical transfer electrode 324. In the vertical line shift, the slope ΔV/ΔT (ΔV denotes a voltage and ΔT denotes time) of rising and falling edges of the vertical drive pulses φV1 to φV4 to be applied to the vertical transfer electrodes 321 to 324 during the horizontal blanking interval Hb, that is, a transient speed (ΔV/ΔT) is made equal to the transient speed (ΔV/ΔT) of the vertical drive pulses φV1 to φV4 to be applied to the vertical transfer electrodes 321 to 324 during the vertical blanking interval, although not shown. FIG. 12B shows rectangular drive pulses having the rising and falling edges perpendicular to the time axis.
It is necessary to perform high-speed vertical transfer during the vertical blanking interval, for example, in correction of camera shaking in CCD video cameras or in a frame interline transfer (FIT) CCDs for broadcasting services.
Vertical line shift that is performed by using four-phase vertical drive pulses during the horizontal blanking interval in CCD imagers is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2000-138943.